1. Field of the Invention
The present invention relates to an antenna and a radio communication device provided with the antenna.
2. Description of the Related Art
When a portable radio communication device is in a communication state, a user's head is located close to the portable radio communication device. In this case, if a radiation pattern of a wave radiated from an antenna provided in the portable radio communication device has a main lobe on a side of the communication device which is close to the user's head, the radiation characteristics of the antenna are greatly varied due to an influence of the user's head, etc., thereon.
As techniques for overcoming such a disadvantage, those disclosed in Jpn. Pat. Appln. KOKAI Publications No. 2002-9534 and No. 2001-339215 are known.
In a portable radio communication device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-9534, an antenna is provided in a housing. The antenna comprises a linear feed element and a linear passive element, which are arranged substantially parallel to each other. The feed element and the passive element extend in a direction perpendicular to the front surface of the housing (which is a surface on which a receiver is provided). The passive element is spaced apart from the feed element in a direction away from the front surface of the housing. To the feed element, current is supplied from feeding means. As a result, the feed element functions as a dipole antenna.
The antenna has a directivity wherein radiation of a wave radiated from the antenna has a peak in a direction from the feed element toward the passive element, due to an operation of a combination of the feed element and the passive element. That is, the antenna has characteristics wherein a radiated wave is directed toward the rear side of the housing, thus reducing the influence of a living body close to the front side of the housing upon the antenna.
Jpn. Pat. Appln. KOKAI Publication No. 2001-339215 discloses an antenna including two feed elements and two passive elements. To be more specific, in the antenna, the two feed elements and the two passive elements are arranged such that the two passive elements are interposed between the two feed elements or the two feed elements are interposed between the two passive elements. Then, currents having opposite phases are supplied to the feed elements, respectively, thereby reducing current flowing through the housing of a radio device, and reducing lowering of the characteristics of the antenna which is caused by an influence of a living body thereon.
However, it is necessary for the antennas disclosed in the above Publications to perform balanced feeding or provide two feeding points, in order to obtain desired radiation characteristics. To carry out balanced feeding, the feeding means needs to include a balun, thus increasing the cost of parts, the loss due to provision of the balun, the area for mounting the parts and the variance in characteristics among manufactured antennas. Also, in the case where two feeding points are provided, the cost of parts, the area for mounting the parts and the variance in characteristics among manufactured antennas increase.
On the other hand, in both a balanced feeding method and an unbalanced feeding method, a loop antenna is known as an antenna in which the variation amount of a radiation pattern is small.
FIG. 22 is a view illustrating the distribution of current at a square 1-wavelength loop antenna. In this type of loop antenna, currents having the same phase are generated at a pair of horizontal elements when the horizontal elements are excited. Thus, as shown in FIG. 23, a horizontally polarized wave is radiated in a direction (X direction) perpendicular to a plane defined by the pair of horizontal elements and a pair of vertical elements. The pair of vertical elements are excited to generate currents having opposite phases at the vertical elements. Thus, as shown in FIG. 23, a vertically polarized wave is radiated in a direction (Y direction) along the horizontal elements. As shown in FIG. 22, current flowing through each of the horizontal elements is larger in value than that of current flowing through each of the vertical elements, and thus the vertically polarized wave is smaller than the horizontally polarized wave.
In such a manner, in the 1-wavelength loop antenna, it is inevitable that a wave greatly radiates in the X direction. In order to restrict radiation of a wave toward the front side of the housing of the portable radio communication device, it is necessary to direct the plane defined by the horizontal elements and vertical elements of the loop antenna in a direction perpendicular to the front surface of the housing. Therefore, the thickness of the housing (i.e., the distance between the front surface and rear surface of the housing) must be sufficiently increased.
FIG. 24 is a view showing the distribution of current at a square 2-wavelength loop antenna. As shown in FIG. 24, in the case where the length of the antenna is set to correspond to two wavelengths, currents having opposite phases are respectively generated at a pair of horizontal elements when the horizontal elements are excited. Also, currents having opposite phases are respectively generated at a pair of vertical elements when the vertical elements are excited. Thus, as shown in FIG. 25, a vertically polarized wave is strongly radiated in the Y direction, and radiation of a horizontally polarized wave in the X direction can be restricted.
Therefore, in the 2-wavelength loop antenna, a plane defined by the horizontal elements and vertical elements is located parallel to the front surface of the housing, and in addition radiation of a wave toward the front side of the housing can be reduced.
However, the 2-wavelength loop antenna occupies a large space in the housing, since its length is great.
In such a manner, conventional antennas have disadvantages in which balanced feeding must be performed or a large space in the housing is occupied by structural elements.
In view of such circumstances, it has been required that an antenna is made small, and in addition reduces radiation of a wave in a specific direction even when unbalanced feeding is performed by using one feeding point only.